β-Lactam susceptibility patterns and investigation of cephalosporin hydrolysing β-lactamases of Gram-negative extraintestinal clinical isolates

Of more than 3500 isolates of enterobacteriaceae, 48–69% were resistant to aminopenicillins and 11–45% to amoxycillin+clavulanic acid. Resistance to second and third generation cephalosporins was present in 11–17 and 3–8% of Escherichia coli , 47–56 and 15–52% of Klebsiella – Enterobacter , 36–57 and 16–27% of Proteus , Providencia and Morganella isolates. Pseudomonas aeruginosa strains varied in their resistance to antipseudomonal β-lactams. Isoelectric points, inhibitor profiles and substrate profiles of β-lactamases extracted from representatives of the resistant strains indicated that the resistance was mainly due to the hyperproduction of chromosomally encoded AmpC β-lactamases. This was confirmed by plasmid profile and PCR investigations. Extended-spectrum β-lactamase and metallo-penicillinase producing strains were not found. One Pseudomonas maltophilia strain produced an oxacillinase.